The present invention relates broadly to a magnetron apparatus and in particular to a rippled-field magnetron apparatus.
In order to achieve efficient conversion of energy from a stream of free electroncs to electromagnetic radiation, near synchronism must be attained between the velocity of the electrons and the phase velocity of the wave. In crossed-field devices, of which the magnetron is a typical example, this synchronism occurs between electrons undergoing a v=E.sub.0 .times.B.sub.0 drift in orthogonal electric and magnetic fields, and an electromagnetic wave whose velocity is reduced by a slow wave structure comprised of a periodic assembly of resonant cavities. This complex system of closely spaced resonators embedded in the anode block limits the conventional magnetron to wavelengths in the centimeter range. Moreover, at high power outputs typical of relativistic magnetrons, rf or dc breakdown in the electron-beam interaction space, and at the sharp resonator edges poses serious problems. The rippled-field magnetron is a novel source of coherent radiation devoid of physical slow-wave structures. Thus, the configuration of the anode and cathode is similar to the so-called "smooth-bore" magnetron. However, it differs from the smooth bore magnetron in that the electrons are subjected to an additional field, an azimuthally periodic (wiggler) magnetic field B.sub.w oriented transversely to the flow velocity v. The resulting -ev.times.B.sub.w force gives the electrons an undulatory motion which effectively increases their velocity, and allows them to become synchronous with one of the fast TE or TM electromagnetic modes (phase velocity .gtoreq.c) characteristic of the smooth-bore magnetron. We note that this technique is also the basis of free-electron lasers (FEL). Thus, in the rippled-field magnetron, the steady state electron motions are governed by well-known magnetron equilibria, but the high frequency wave instability which leads to the sought-for radiation is determined by a free electron laser like, parametric interaction. The device differs from the free electron laser in that the electron source (the cathode) and the acceleration region (the anode-cathode gap) are an integral part of the rf interaction space. This makes for high space-charge densities and for large growth rates of the free electron laser instability. Typically, the magnetron configuration is cylindrical rather than linear as in conventional free electron lasers, and the system is therefore very compact. The cylindrical geometry also allows for a continuous circulation of the growing electromagnetic wave and thus the system provides its own internal feedback. Therefore, the rippled-field magnetron is basically an oscillator rather than an amplifier as is the case of the free electron laser.